Crosstalk reduction in parsitically coupled circuits

ABSTRACT

The amplification of crosstalk between two isolated circuits due to a resonant condition between the two circuits is reduced by coupling the electrical grounds associated with the circuits with a series Resistor-Capacitor network.

CROSS REFERENCES

This is a continuation of application Ser. No. 08/904,520, now U.S. Pat.No. 5,861,783, filed on Aug. 1, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to crosstalk reduction in electrical circuits andin particular to crosstalk reduction in communication circuits.

2. Description of the Related Art

In many communication systems electrical communication signals areconveyed between various equipment used in such systems. Typically, theelectrical signals can be interconnected and routed to various equipmentlocated at a central office. The interconnection and rerouting of theelectrical signals are implemented with interconnection modules, some ofwhich are called Digital Distributing Frames (DDF).

In a central office location which may contain various communicationequipment, a plurality of DDF modules are typically disposed in ahousing. Typically, a DDF module comprises several connectors, such asBNC connectors mounted on a printed circuit board. Cables carrying theelectrical signals can be connected to the mounted connectors of thevarious housed DDF modules and to the various equipment at the centrallocation thus interconnecting and rerouting various electrical signals.As is typically used with printed circuit boards, conductorselectrically connected to the mounted connectors are formed on thecircuit board by well known etching or plating processes. The conductorsare typically traced on the surface of the printed circuit board so asto electrically and physically isolate them from other conductors tracedon the printed circuit board. Each of the conductors may be associatedwith a particular electrical circuit and mounted connector. Theparticular circuit may be represented by conductor traces on the printedcircuit board that may or may not be connected to components (e.g.,resistors, capacitors, diodes, transformers) mounted on the printedcircuit board. Also, each particular electrical circuit has its ownelectrical ground which is substantially electrically and physicallyisolated from other electrical grounds and conductor traces on theprinted circuit board. The electrical and physical isolation of theconductors help prevent crosstalk between conductors associated withdifferent circuits. Crosstalk is defined as undesired electromagneticcoupling that occurs between proximately located conductors carryingelectrical signals.

Current DDF modules support two isolated circuits each of which iselectrically connected to a mounted connector. These modules may alsohave one or two other connectors for monitoring the isolated circuits.Conductor traces connected to the various mounted connectors associatedwith the isolated circuits may be in close proximity to each other. As aresult crosstalk can occur between these traces. However, said crosstalkcan be attenuated with electrical shielding structures (e.g., platedvias, ground planes) mounted on the printed circuit board. Thesestructures tend to further isolate the traces, including electricalground traces, associated with the isolated circuits thereby reducingcrosstalk that may occur between these circuits. However, under somecircumstances, a resonant condition occurs between isolated circuitswhich can greatly amplify any crosstalk between such circuits renderingthe electrical shielding technique ineffective.

Typically an electrical circuit such as the isolated circuits has anatural frequency which is the frequency at which a circuit oscillatesin response to an impulse signal. The isolated circuit may also havecircuit loops which are electric paths comprising of various electricaldevices including conductors and parasitic elements. A parasitic elementis an unintended element that is created as a result of the geometry ofan electrical circuit. Thus, a resonant condition occurs when a signalis applied to one or both of the isolated circuits such that the signalfrequency matches the natural frequency of unintended circuit loopsformed by the parasitic elements. Therefore, a resonant condition occursat certain frequencies or group of frequencies.

Methods are currently available to reduce the effects of such resonantconditions, but such methods result in the degradation of the signalsapplied to the isolated circuits. For example, the introduction of loss(connecting a resistor in series with the applied signals) to theisolated circuit can reduce crosstalk but such a method will adverselyeffect the signal quality (e.g., amplitude, phase and frequencydistortions).

Therefore, there exists a need to reduce crosstalk due to resonatingisolated circuits which are traced on a printed circuit board.

SUMMARY OF THE INVENTION

The present invention provides for a resistor-capacitor network thatcouples the respective electrical grounds of two circuits that areresonating with each other and to which signals having a certain qualityare applied thus reducing crosstalk due to the resonant conditionbetween the circuits substantially without degrading the quality of theapplied signals.

The resistor-capacitor network comprises a resistor connected to thefirst electrical ground and a capacitor connected to the resistor and tothe second electrical ground. The capacitor allows a current to flowthrough the resistor during a resonant condition thereby reducing thecrosstalk due to resonance between the circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a perspective view of the present invention;

FIG. 2 depicts one layer of the multilayered printed circuit board ofthe present invention having two isolated ground planes;

FIG. 3 shows an electrical schematic of the present invention;

FIG. 4 shows a side view of the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a perspective view of the present invention which is aDDF module comprising a printed circuit board with layers 116, 118 and120. Although three layers are shown, the printed circuit board is notlimited to any specific number of layers. The layers are typically madefrom an insulating material (e.g., FR-4, Epoxy Glass, Teflon) materialwhich has a certain dielectric constant. A resistor 106 connected inseries with a capacitor 108 serves to couple the electrical grounds 134,136 (see FIG. 2) associated with a first and second circuit on theprinted circuit board. Signals having a certain signal quality areapplied to one or both of the isolated circuits. The signal quality istypically viewed as the state of the frequency components, amplitude andphase of a signal. For example, a signal of poor quality may havedistorted frequency components, a distorted amplitude or phase. Atfrequencies where a resonant condition occurs, capacitor 108 allowscurrent to flow through resistor 106. Resistor 106 by its very naturedissipates the energy of the flowing current. Thus, at resonantfrequencies, current flows from one ground with which a first circuit isassociated to another ground with which a second circuit is associatedallowing resistor 106 to dissipate the energy of the flowing currentthereby substantially eliminating the amplifying effect of the resonantcondition on crosstalk substantially without degrading the quality ofthe applied signals. Typically, since the energy associated with thecrosstalk at resonance is substantially smaller than the energyassociated with the applied signals, the resistor has an insignificanteffect on signal quality.

The connectors shown are BNC connectors but other similar connectors(e.g. SMA connectors) used for coupling electrical signals to a printedcircuit board can be used. Connectors 100 and 104 are associated with afirst circuit for a first signal type (e.g. input signals).

Connectors 100 and 104 are compliant pin type BNC connectors; connectors100 and 104 have pins 102 which frictionally engage with holes 101 ontop layer 116 (and on bottom layer 120; not shown) such that theconnectors are fixedly attached to the printed circuit board. Centralconductors 122 (not shown) of connectors 100 and 104 engage with platedthrough holes 124 on top layer 116 and bottom layer 120. Although notshown, it will be understood to one of ordinary skill in the art towhich this invention belongs that the central conductors 122 ofconnectors 100 and 104 are electrically connected to each other via anetched conductor on one of the printed circuit board layers (116, 118and 120). Any one of the printed circuit board layers (116, 118 and 120)can be used to electrically connect these central conductors. Theparticular layer used to electrically connect central conductors 122 ofconnectors 100 and 104 depends on specific design requirements of theDDF module.

A coaxial cable (not shown) carrying, for example, an input signal, istypically connected to connector 100. Another coaxial cable (not shown)having two ends has one end connected to connector 104 and the other endconnected to another similarly configured DDF module or othercommunication equipment configured to receive such a connector. Thus,the first signal type associated with a first circuit can be routed orinterconnected to other DDF modules or other equipment.

Although not shown, connectors 126, 130 and 132 have a central conductor122. Connectors 126, 128, 130 and 132 are similarly compliant pin BNCconnectors whose central conductors 122 are coupled to an electricalsignal via coaxial cables (not shown). Connectors 126 and 128 areassociated with a second circuit for a second signal type (e.g., outputsignals). Central conductors 122 of connectors 126 and 128 areelectrically connected to each other via an etched or plated electricalnetwork on one of the layers (116, 118 and 120). A coaxial cablecarrying the second signal type is connected to connector 128. Thesecond signal type can be routed and/or interconnected to otherequipment or another DDF module via another coaxial cable connected toconnector 126.

Connectors 130 and 132 are monitor connectors which are used to monitorthe first and second signals respectively. In particular, centralconductor 122 (not shown) of connector 130 engages a plated through hole124 which is electrically connected, via an etched or plated electricalnetwork on one of the printed circuit board layers (116, 118 and 120),to the plated through hole 124 for connector 100. Thus, when connector130 is mounted on the printed circuit board, its central conductor 122is electrically connected to the central conductor 122 of connector 100through an electrical network. Consequently, a coaxial cable can beconnected to connector 130 and to an equipment that can monitor thefirst signal. Connector 132 is similarly mounted to the printed circuitboard such that its central conductor 122 is electrically coupled to thecentral conductor of connector 126. Thus, the second signal can bemonitored with monitoring equipment connected to connector 132 via acoaxial cable. It should be noted that other cables (not only coaxialcables) can be used to couple electrical signals to the mountedconnectors.

Referring to FIG. 2, there is shown a top view of second layer 118.Electrical ground conductor 134 associated with the first circuit andelectrical ground conductor 136 associated with the second circuit areshown. The electrical grounds (134, 136) are typically a metallicsurface etched or plated onto layer 118 which has a particulardielectric constant. The shapes of the electrical grounds depicted arefor illustrative purposes only. The particular geometry of theelectrical grounds depends on the particular design requirements of therespective electrical circuits with which they are associated.

Referring to FIG. 3, there is shown a schematic of the present inventionin which a series Resistor-Capacitor network serves to electricallycouple the two electrical grounds thereby reducing or substantiallyeliminating amplification of crosstalk due to a resonant conditionbetween the two circuits with which the electrical grounds areassociated. The series Resistor-Capacitor network comprises resistor 106and capacitor 108 electrically connected to each other via conductor114. Conductor 112 connects resistor 106 to electrical ground 134.Conductor 110 connects capacitor 108 to electrical ground 136. Theparticular value of resistor 106 and capacitor 108 can vary depending onthe particular frequency at which the resonant condition occurs. Also,additional resistors and capacitors may be added to the series networkto reduce or substantially eliminate the amplifying effect on crosstalkof a resonant condition between the two circuits.

FIG. 4 depicts a side view portion of the DDF showing how the twogrounds are coupled with the series Resistor-Capacitor network.Electrical grounds 134 and 136 are etched or plated onto layer 118.Resistor 106 is a surface mount resistor which is electrically connectedto conductor 112 and conductor 114. Conductor 112 is in turnelectrically connected to electrical ground 134 via plated through hole138. Capacitor 108 is also connected to conductor 114 and to conductor110 which is in turn connected to electrical ground 136 via platedthrough hole 140. Although the series Resistor-Capacitor network isimplemented with surface mountable components, it will be understood tothose of ordinary skill in the art to which this invention belongs thatother type of components (e.g., discrete, through hole, thin film,integrated circuits ) can be used. Even though the present inventiondiscusses DDF modules with two different circuits, it will be understoodthat the present invention is applicable to DDF modules with more thantwo different circuits. It will be further understood that the presentinvention is not limited to electrical circuits on DDF modules but toany circuitry on any type of printed circuit board which experiencescrosstalk amplification as a result of a resonant condition involvingparasitic elements between at least two different circuits on a printedcircuit board.

I claim:
 1. A resistor-capacitor network for reducing crosstalk due to aresonant condition between a first circuit having a first electricalground and a second circuit having a second electrical ground wheresignals of a certain quality are applied to the circuits, theresistor-capacitor network comprising:a resistor connected to the firstelectrical ground; and a capacitor connected to the resistor and to thesecond electrical ground where the first and second electrical groundsare mounted on a layer of insulating material; the capacitor allowing acurrent to flow from the first electrical ground to the secondelectrical ground and through the resistor during a resonant conditionthereby reducing the crosstalk due to resonance between the circuitssubstantially without degrading the quality of the applied signals. 2.The resistor-capacitor network of claim 1 wherein the resistor andcapacitors have values which are varied depending on a particularfrequency at which the resonant condition occurs.
 3. Theresistor-capacitor network of claim 1 wherein the resistor and thecapacitor are surface mountable components.
 4. The resistor-capacitornetwork of claim 1 wherein the resistor and capacitor are discretecomponents.
 5. The resistor-capacitor network of claim 1 wherein theresistor and capacitor are thin-film components.
 6. Theresistor-capacitor network of claim 1 wherein the resistor and capacitorare integrated circuits.
 7. A DDF module comprising:a printed circuitboard having at least one layer; a first circuit and a second circuitmounted on the printed circuit board; a first electrical groundassociated with the first circuit and a second electrical groundassociated with the second circuit are etched onto the at least onelayer of the printed circuit board; a resistor-capacitor network thatcouples the electrical grounds allowing a current to flow from the firstelectrical ground to the second electrical ground thereby reducing anycrosstalk between the first and second circuit.
 8. The DDF module ofclaim 7 where the electrical grounds are etched onto the at least onelayer of the printed circuit board.
 9. The DDF module of claim 7 furthercomprising more than two different circuits.
 10. A DDF modulecomprising:a first connector for coupling to a cable carrying a firstsignal; a second connector for coupling to a cable carrying a secondsignal; a first monitor connector for monitoring signals at the firstconnector; a second monitor connector for monitoring signals at thesecond connector; a first circuit having a first electrical ground iscoupled to the first connector; a second circuit having a secondelectrical ground is coupled to the second connector; and aresistor-capacitor network coupling the electrical grounds allowing acurrent to flow from the first electrical ground to the secondelectrical ground so as to reduce crosstalk between the first and secondsignals.
 11. The DDF module of claim 10 where the connectors are BNCconnectors.
 12. The DDF module of claim 10 comprising additionalconnectors for coupling to cables carrying different signals where atleast one monitor connector is coupled to at least one of the additionalconnectors.
 13. A circuit for reducing crosstalk, the circuitcomprising:a resistor-capacitor network having at least one resistorcoupled to at least one capacitor; the resistor-capacitor networkcoupling different grounds of different electrical circuits allowing acurrent to flow between the grounds so as to reduce crosstalk betweensuch circuits.
 14. The circuit of claim 13 wherein theresistor-capacitor network reduces crosstalk between circuits in which aresonant condition exists.
 15. The circuit of claim 13 wherein theresistor-capacitor network couples an electrical ground of a firstcircuit to an electrical ground of a second circuit.